Method of making a heat sinked resistor

ABSTRACT

A ceramic core has selected resistance alloys wound thereon, and is covered by a vitreous-enamel insulating layer, and finally by a relatively thick metallic outer covering that is bonded to the vitreous-enamel layer. The metallic outer covering can be applied by pouring molten metal over the vitreous-enamel covered resistor that has been preheated. The resultant electrical resistor is designed for the utmost stability under extreme operating conditions, and has a power rating that is increased over fivefold from the power rating without the metal outer layer.

[ Nlar.5,1974

United States Patent- [191 Shirn METHOD OF MAKING A HEAT SINKED 8/1947-Deyrup 11/1949 RESISTOR I Kohring 12/1950 Scott et al..

[75] Inventor:

George A. Shirn, Williamstown, Mass.

[73] Assignee: Sprague Electric Company, North FOREIGN PATENTS ORAPPLICATIONS Adams, Mass.

Apr, 5, 1972 1/1948 Great Britain..l...................,.

[22] Filed:

Primary Examiner-Charles W. Lanham Assistant ExaminerVictor A. DiPalma[21] APPL NOQ 241,420 I Related U.S. Application Data [63]Continuation-impart of Ser. No. 104,621

' [57] ABSTRACT A ceramic core has selected resistance alloys woundthereon, and is covered by a vitreous-enamel insulat- Jan. 7,

1971 abandoned.

ing layer, and finally by a relatively thick metallic outer coveringthat is bonded to the vitreous-enamel layer. The metallic outer coveringcan be appliedby pouring molten metal over the vitreous-enamel 'cov-338/269, 29/613 H01c 3/00, HOlc 5/00, H010 17/00 [51] Int. Cl.

0 5 6 6 d 1 6 E a 63 ,4 m Km 92 2 5 1 .5 "W $3 .m .3 f o d l .m F. .l 85 References Cited UNITED STATES PATENTS 5 1912 Barrmger 29/618 x 4(Ilaims, 2 Drawing Figures 111932 Terwilliger..........................338/262 METHOD OF MAKING A HEAT SINKEI) RESISTOR This is acontinuation-in-part of application Ser. No.

into the metal 104,621, filed January 7, 1971 and later abandoned.BACKGROUND OF THE INVENTION core upon which the resistance wire iswound, or a ceramic core thatis plated or is impregnated, and a coatingover the resistance and the core to aid in electrically insulating anddispersing the heat of the resistor, as well as protecting the resistor.When the resistor develops high temperatures under electrical loads, theheat transfers to the coating and dissipates into the atmosphere and, asis the case with heat sinked resistors, chassis upon which the resistoris mounted. I I

Different materials for surrounding resistors have been tried withvarying degrees of success. The coating or housing materials should havethe qualities of .electrical insulation, heat dissipation, thermalprotection, humidity protection, maintaining solidification and shockresistance, break-proof, andbe economical to use in the fabrication ofthe resistors. While some coatings or housings solve certainrequirements, no treatment her'ebefore used has been completelysatisfactory.

Sealing resistors in glass is economical, affords high temperature andmoisture protection, but produces fragile units that are not suited forparticularly heat sensitive elements. Silicone varnish dip-coatings havegood thermal stability, good moisture resistance, fair solventresistance, but are frequently difficult to apply, subject to pinholes,and thus electrical insulation failures, and are not good thermalconductors. Silicone polymers, highly filled with inorganic materialssuch as alumina, mica, asbestos, silica or metallic oxides ordinarilyhave good thermal stabiiity,'fair moisture resistance, but have poorsolvent resistance and manufacturing difficulties associated withmaintaining the tiller in suspension and in proper ratio.

Sleeves or tapes of various polyesters, fluorocarbons, glass fibers,phenolics, and silicones, alone or in combination are expensive toapply, are good electrical insulators, withstand thermal shock, but arepoor thermal conductors. These sleeved or taped resistors can beinserted into metallic housings, or thin sleevings such as aluminum, toincrease the heat dissipation, but at a greater manufacturing cost, andwith attendant difficulties in cente'ring the resistor in the housingandsubsequently filling the remaining space. v

Ceramic tubes have high temperature humidity, and solvent protection,but are difficult to seal hermetically and are susceptible to cracks orbreaks due to the brittleness of the ceramic.

Metal housings for wire-wound resistors have the excellentcharacteristics necessary to provide reliable and stable resistors.HOwever. these units are produced at rather high manufacturing costs,and have heretofore been difficult to work with because of cumbersomecements and/or cement-fillers. Some have mixed beryllia flour with anexposy filler, but beryllia flour can be quite dangerous to work with.Prior art metal housings that were cast on the resistor cores havesuffered from poor insulation characteristics and are subject tocracking upon the application of the cast metal housing.

Accordingly, it is an object of the present invention to provide anelectrical resistor that has good electrical insulation properties, aswell as the ability to dissipate heat evenly and properly.

A further object of this invention is to provide a resistor that ismoisture-proof and solvent resistant.

A still further object of the present invention is to provide a means ofeconomically manufacturing a resistor possessing good electricalinsulation characteristics that will not crack upon the application of acast metal outer coat. I

SUMMARY OF THE INVENTION Leads can be welded to extend out of oppositeends of t .ends of the vitreous-enamel and the leads. The mold can bedesigned in such a manner that tabs are provided for attaching thehousing to a chassis, and optionally the surface area of the metalhousingcan be increased for cooling purposes by use offins" or flangesthereon.

A vitreous-enamel covered resistor that had a power rating of 5 wattsprior to being cast in a metal housing as described herein istransformed into a heat sinked resistor whose power rating is increasedover five-fold by casting same in an aluminum housing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded view of theresistor of the present invention; and FIG. 2 is a perspective view ofthe assembled resistor of FIG. I.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I illustrates a heat sinkedwire-wound resistor within the scope of this invention. The resistorcore 10 is generally composed of a substantially smooth ceramicmaterial. for example, percelain, magnesium sili' cate or magnesiumaluminum silicate. The resistor core 10 is wound with a bare or uncoatedelectrical-resistance wire 11, in direct contact therewith byconventional winding means. The wire 11 would normally be composed of aniron-chromium-aluminum alloy. Nickel-chromium alloys or nickel-copperalloys can be utilized as the wire I1. The wire 11 is securely held inplace by means of nickeled steel end caps 12 or other type of endterminals which are thermally shrunk or I swaged in place to hold themto the ends of the resistor core 10 and to which the ends of the wire 11are welded to form an electrically conducting unit from one end cap 12through the wire 11 to the other end cap. Bare terminal leads 13 areconnected to each cap 12 as. by welding, brazing or soldering with hightemperature solder, so as to extend outward therefrom.

This unit is then insulated by dipping it in a clay slip and drying itin a normal atmosphere. After drying, the unit is fired in a kiln heatedto approximately 700 C in order to fuse the enamel 14. This process canbe repeated as many times as necessary to achieve a suitable thickness.it is repeated three times herein to obtain the proper amount ofinsulation. This vitreous-enamel covering should have an expansioncoefficient that is closely matched to that of the ceramic core in orderto insure positive protection of the resistance wiring.

Another method of applying the vitreous-enamel coating is to flame-spraythe solid coatings by feeding the powders into a powder flame-sprayburner. Other suitable methods are available if desired, and may be usedwithout limiting the scope of this invention.

The vitrous-enamel layer serves to insulate the turns of bare wire fromone another and secure the turns in position in proper spacing on thecore. The vitreous layer also provides a continuous barrier against thepenetration of the outer metal covering between the turns of thewinding. The fusing .of the enamel layer acts to produce a substantiallylevel coating inwhich the elevations'of the turns of the winding areobscured, so that they are not reflected in the outer metal covering.'

This vitreous-enamel protected resistor is now capable of being coveredby a layer of aluminum that can be applied in the molten state in apreheated mold.

The aluminum housing of FIG. 2 is cast on the vitreous-enamel 14 byplacing the vitreous-enamel covered resistor into a mold capable ofwithstanding relatively high temperatures, preheating the unit to atemperature of approximately 350 C, and pouring molten aluminum into themold that is designed in such a fashion that the aluminum 15 willsurround substantially all of the enamel covered resistor 14 leaving theoutermost portion 18 of the caps 12 exposed. The resistor 20 is thencooled and removed from the mold. The resistor 20 can be polished ifnecessary to remove any rough edges on the outer surface thereof.

The temperature of the mold and enamel. covered resister is raised to350 C before aluminum is poured therein to facilitate and ensure asmooth even flow of the molten metal through the mold, and to preventthe vitreous-enamel coating 14 from cracking from thermal shock upon theaddition of the molten metal. Aluminum melts at approximately 660 C andflows freely through a mold preheated to 350 C. A bond is formed betweenthe molten metal and the vitreous-enamel, resulting in a compact,stable, protective coating for the resistor, thereby giving the unit abetter heat conducting path than electrical resistors have. been able toachieve in prior art resistors.

The mold and resistor unit should be preheated to a temperature notgreater than 500C, but at least to that point where the molten metal canflow smoothly through the mold without solidifying or cracking thevitreous-enamel. if the temperature is allowed to go above 500 C, thevitreous-enamel coating will start to liquify and the metal will come incontact with the woundwire.

allowing for the main heat sinking Optionally, the design of the moldmay be such as to provide for cooling flanges 16 on the metal housing.The flanges 16 give more surface area to the housing and allows for amore rapid and complete dissipation of heat into the atmosphere. Thedesign of the mold also should provide for mounting tabs 17 so that theflat-bottomed resistor can be secured to a metal chassis contact to themetallic chassis. I 7

it should be emphasized that the vitreous-enamel insulating layer andthe precaution of casting the aluminum in a preheated mold permit thedesirable results achieved herein.

While aluminum is mentioned as the preferredv metal herein, other highthermally conductive materials may be used with varying degrees ofsuccess. Some of these other metals include lead, tin, zinc,'magnesium,and alloys or mixtures thereof.

Any metal used should melt below approximately 800 C, as the metal mustbe heated about 50 C higher than its melting point to be able to flowfreely. If the temperature of the metal is too high-beyond 800-900C'-then the vitreous-enamel coating will crack or disintegrateuponexposure to these temperatures, or the molten metal may solidify when itcomes in contact with the relatively cooler mold; As was prising;

pointed out earlier, the mold should not be heated beyond 500 C. So anyhighly thermally conductive metal,

or alloy, can be used if it melts below 800 C.

While heat sinked casting of the metal is preferred herein, it is notthe only method that can be used.0ther methods giving similar, althoughless desirable, results include die casting, spraying, and powdersintering; all are well known methods to those skilled in the art.

A resistor thusly formed is thermally and physically more stable thanprior art resistors; it is protected from corrosive fumes mechanicaldamage, and the effects of humidity. As mentioned earlier herein, theaddition of the outer metal layer increases the power rating of thevitreous-enamel covered resistor at least five fold. The heat conductingpath formed thereby, gives an even dissipation of heat that allows forexcellent quick cooling of the resistor unit. 7 V

What is claimed is:

1. The method of forming an electrical resistor coma. winding a bareresistance wire along a substantially smooth ceramic core, said. wirebeing in direct contact with said core;

b. placing metal terminals on opposite ends of said core and connectingsaid wire to said terminals to form an electrically conducting unit;

c. applying a vitreous-enamel insulating layer over said conducting unitso as to insulate. the turns of said bare wire from one another andsecure the turns onto said cor'e, said layer having a thickness toprovide a continuous barrier against penetration of an outer metalcovering between the turns'of said winding, said layer having anexpansion coefficient closely matched to said core;

d. firing said insulated conducting unit at approximately 700 C to fusesaid vitreous-enamel layer; and I e. applying a thermally conductivemetal outer cover ing having a melting point below 800 C oversubstantially all of said insulated conducting unit in direct contactwith said insulating layer, said coverto be through -4. A method offorming an electrical resistor comprising: I

a. winding a bare resistance wire along a substantially smooth ceramiccore, said wire being in direct contact with said core;

b. placing metal terminals on opposite ends of said core and connectingsaid wire to said terminals to form an electrically conducting unit,said terminals having bare leads extending outward therefrom;

c. applying a vitreous-enamel insulating layer over said conducting unitso as to insulate the turns of said bare wire from one another andsecure the turns onto said core, said layer having a thickness toprovide a continuous barrier against penetration of an outer metalcovering between the turns of said winding, said vitreous-enamel layerhas an expansiori coefficient that is closely matched to that of saidceramic core;

d. firing said insulated conducting unit at approximately 700 C to fusesaid vitreous-enamel layer;

e. preheating saidunit in a mold to a temperature that will allow forthe free flow of molten metal therethrough, but not to exceed 500; and

f. casting molten aluminum thereon substantially covering said unit andleaving said bare terminal leads out of contact with said aluminum.

1. The method of forming an electrical resistor comprising: a. winding abare resistance wire along a substantially smooth ceramic core, saidwire being in direct contact with said core; b. placing metal terminalson opposite ends of said core and connecting said wire to said terminalsto form an electrically conducting unit; c. applying a vitreous-enamelinsulating layer over said conducting unit so as to insulate the turnsof said bare wire from one another and secure the turns onto said core,said layer having a thickness to provide a continuous barrier againstpenetration of an outer metal covering between the turns of saidwinding, said layer having an expansion coefficient closely matched tosaid core; d. firing said insulated conducting unit at approximately700* C to fuse said vitreous-enamel layer; and e. applying a thermallyconductive metal outer covering having a melting point below 800* C oversubstantially all of said insulated conducting unit in direct contactwith said insulating layer, said covering being applied by preheatingsaid unit in a mold to a temperature that allows for the free flow ofmolten metal therethrough but not exceeding 500* C, and casting saidcovering thereon in its molten state.
 2. The method of claim 1 whereinsaid mold is designed so as to provide cooling flanges on the outersurface of said metal covering and mounting tabs attached to the basethereof.
 3. The method of claim 1 wherein said metal outer covering isof aluminum.
 4. A method of forming an electrical resistor comprising:a. winding a bare resistance wire along a substantially smooth ceramiccore, said wire being in direct contact with said core; b. placing metalterminals on opposite ends of said core and connecting said wire to saidterminals to form an electrically conducting unit, said terminals havingbare leads extending outward therefrom; c. applying a vitreous-enamelinsulating layer over said conducting unit so as to insulate the turnsof said bare wire from one another and secure the turns onto said core,said layer having a thickness to provide a continuous barrier againstpenetration of an outer metal covering between the turns of saidwinding, said vitreous-enamel layer has an expansion coefficient that isclosely matched to that of said ceramic core; d. firing said insulatedconducting unit at approximately 700* C to fuse said vitreous-enamellayer; e. preheating said unit in a mold to a temperature that willallow for the free flow of molten metal therethrough, but not to exceed500*; and f. casting molten aluminum thereon substantially covering saidunit and leaving said bare terminal leads out of contact with saidaluminum.